The need for treating sewage sludge has increased substantially in recent years. Untreated sludge, both in solid and liquid form, may contain any number of noxious substances that are harmful to humans and the environment. These include particulate solids, organic and inorganic compounds, and pathogens.
There is developing an increasing demand, particularly with respect to municipal sewage sludge, that the treatment of sludge be sufficiently thorough to allow beneficial re-use of sludge, such as enabling the sludge to be applied to land on which agricultural and ornamental crops may be grown. While sewage sludge has beneficial plant nutrients, it also may contain bacteria, viruses, protozoa, parasites, and other microorganisms which may be disease causing. In an effort to make stabilized sludge more marketable, those treating the sludge have attempted to produce a granular, scatterable product having a soil-like texture.
One way of treating such sludge is to mix alkaline products with the sludge in a manner that blends the alkaline and sludge and transforms the otherwise watery sludge into an acceptable end product. Where this method is used without sufficient increase in sludge temperature, the resulting material is only partially stabilized. Higher pathogenic organisms are not destroyed. Sludges stabilized in this manner may be suitable for land application, but are subject to significant regulations relating to public access, harvesting and grazing.
One further method to treat waste to kill pathogens is to heat it to a high temperature for a period of time. Commonly known as pasteurization, this process neutralizes pathogens to a degree dependent upon the level of temperature and length of time that the waste is exposed to the elevated level. Where both pasteurization and the addition of an alkaline added in combination are performed, pasteurization temperatures can kill pathogenic organisms while the alkaline additive can prevent regrowth of organisms. The result can be an end product that can be stored for extended periods of time. If satisfactorily processed in this manner, sludge may be applied to lands without need for site-specific permits, according to current federal regulations, and, thus may be marketed, distributed, and sold as fertilizer.
In order to properly ensure complete pathogen reduction, the heated sludges must be held at elevated temperatures for a specific period of time. The U.S. Environmental Protection Agency has created standards which these sludges must maintain. Since these sludges are exempt from many restrictions for land application, EPA has stated:
The time-temperature requirements apply to every particle of sewage sludge processed. Time at the desired temperature is readily determined for batch operations, turbulent flow in pipes, or even monitored flow in pipes. USEPA/625/R-92/013, December 1992 (emphasis added).
The U.S. Environmental Protection Agency recognizes that the accurate measurement of time-temperature is easily accomplished in dealing with sludges of liquid consistencies, which flow through pipes, such as 0-5% dry solids content, or in batch type operations. Some difficulty may lie in ensuring the proper treatment time for sludge that has been dewatered to a solids content of approximately 10-60%, which behaves more like a solid than a liquid.
Conveying and mixing waste water sludges are difficult operations due to certain inherent characteristics of waste water sludges. Some such characteristics are the thixotropic nature of sludges, the variations in the sludge characteristics depending upon the process or methods that may be used in the waste water process, the type of sludge that is produced as a function of the community, variations in the amount of solids present in the sludge, and variations in the properties and characteristics of the alkaline materials that are used for stabilization of sludge in those instances where alkaline materials are used.
More particularly, biological sludges, particularly sewage sludges, are thixotropic. These sludges, when agitated, have the tendency to liquify. The agitation provides added mechanical energy to the sludge in order to mix the sludge with the alkaline material. Further agitation adds mechanical energy in order to move or convey the mixture through the machinery or to transport the material from one location to another. Agitation can mechanically add energy in a further attempt to granulate or break up the sludge. Sludges when exposed to such agitation become more liquid. The watery material is difficult to handle and becomes less desirable to use for agricultural purpose.
In the past, the primary method of preventing sludge from becoming watery is by the use of large quantities of alkaline materials. The present invention provides for a highly efficient use of alkaline material. The previous methods have consisted of blending waste sludge with huge quantities of alkaline material and then discharging the material into an open pile on the ground. These methods rely on the use of large quantities of alkaline materials for two primary reasons. First, the larger quantities of alkaline material are necessary to maintain a granular consistency. Second, the large quantity of alkaline material is necessary to heat the sludge sufficiently in order to overcome the tremendous heat loss that results from dumping the material into an open pile.